1. Field of the Invention
The present invention relates to a signal-to-noise enhancer, and more particularly, to a signal-to-noise enhancer which has a limiter for limiting an amplitude of a main signal in an input signal, which improves a ratio of the main signal to noise (S/N) in the input signal, and which is used for receiving, for example, a satellite broadcasting signal.
2. Description of the Prior Art
An example of the conventional signal-to-noise enhancers is disclosed in Japanese Patent Provisional Publication No. 123502/1992. FIG. 11 is a circuit diagram showing an example of the conventional signal-to-noise enhancers. The signal-to-noise enhancer 1 shown in FIG. 11 includes an input terminal 2. The input terminal 2 is connected to an input end of a directional coupler 3 used as a divider. The directional coupler 3 divides the input signal applied to the input terminal 2 into a high-level signal, which is almost the same in level as the input signal, and a low-level signal which is at a lower level than the input signal by, for example, 30 dB.
Two output ends of the directional coupler 3 are connected to inputs of two magnetostatic wave filters 4 and 5 utilizing the magnetostatic surface wave mode, respectively. These magnetostatic wave filters 4 and 5 have the same frequency-selective nonlinear limiting characteristics. The frequency-selective nonlinear limiting characteristic is a characteristic wherein a signal at a frequency f1 is limited in excess of a saturation level, while a signal at a frequency f2, which is different from the frequency f1, having the saturation level or less is not limited. That is, the frequency-selective nonlinear limiting characteristic is a characteristic wherein when a signal is over a saturation level, a saturation operation is generated, and limiting by the saturation operation is individually generated at each frequency. One magnetostatic wave filter 4 is used as a limiter for limiting an amplitude of a high-level main signal in the high-level signal obtained from the directional coupler 3. The other magnetostatic wave filter 5 is used for passing the low-level signal obtained from the directional coupler 3.
An output of the magnetostatic wave filter 4 is connected to an input of an attenuator 6. The attenuator 6 is for attenuating a level of a signal obtained from the magnetostatic wave filter 4. An output of the other magnetostatic wave filter 5 is connected to an input of a delay line 7. The delay line 7 delays a phase of a signal obtained from the magnetostatic wave filter 5. An output of the attenuator 6 and an output of the delay line 7 are connected to two inputs of a directional coupler 8 used as a combiner, respectively. The directional couple 8 is for attenuating a level of a signal obtained from the attenuator 6, and is for combining the signal having the attenuated level and a signal obtained from the delay line 7. Finally, an output of the directional coupler 8 is connected to an output terminal 9.
Thus, in the signal-to-noise enhancer 1, between the input terminal 2 and the output terminal 9, a first path is composed of the directional coupler 3, the magnetostatic wave filter 4, the attenuator 6 and the directional coupler 8, and a second signal path is composed of the directional coupler 3, the magnetostatic wave filter 5, the delay line 7 and the directional coupler 8.
In the signal-to-noise enhancer 1, when an input signal including a high-level main signal and a low-level noise different from the main signal in frequency is applied to the input terminal 2, the input signal is divided by the directional coupler 3 into a high-level signal which is almost the same in level as the input signal and a low-level signal which is less in level than the input signal by, for example, 30 dB. In this case, the high-level signal includes a high-level main signal and a low-level noise different from each other in frequency, the low-level signal includes a low-level main signal and a lower-level noise different from each other in frequency.
In the magnetostatic wave filter 4, though the main signal in the high-level signal is limited because the main signal has a high-level, the noise in the high-level signal is not limited because the noise is different from the main signal in frequency and has a low-level. On the other hand, in the other magnetostatic wave filter 5, the main signal and the noise in the low-level signal are not limited at low-levels. Meanwhile, due to insertion loss in the magnetostatic wave filters 4 and 5, the high-level signal and the low-level signal are slightly attenuated in level, respectively.
A level of a signal obtained from the magnetostatic wave filter 4 is attenuated by the attenuator 6, a phase of a signal obtained from the other magnetostatic wave filter 5 is delayed by the delay line 7. Then, in the directional coupler 8, a level of a signal obtained from the attenuator 6 is attenuated, and the signal having the attenuated level and a signal obtained from the delay line 7 are combined. In this case, the level of the signal obtained from the magnetostatic wave filter 4 is attenuated by the attenuator 6 and the phase of the signal obtained from the magnetostatic wave filter 5 is delayed by the delay line 7 so that noise in the two signals combined by the directional coupler 8 are at the same level and opposite in phase. Thus, the noise from the first signal path, including the magnetostatic wave filter 4, and the noise from the second signal path, including the magnetostatic wave filter 5, offset each other in the directional coupler 8. Also, though the main signal from the first signal path is limited by the magnetostatic wave filter 4, the main signal from the second signal path in not limited by the magnetostatic wave filter 5. Thus, from the output of the directional coupler 8 and the output terminal 9, a main signal having a level corresponding to the limited level is obtained. Accordingly, in the signal-to-noise enhancer 1, the signal-to-noise ratio in the input signal is improved.
Also, in the signal-to-noise enhancer 1, the minimum level of the input signal wherein the signal-to-noise ratio is improved is -12 dBm, the level of the input signal wherein it begins to improve the signal-to-noise ratio is -19 dBm, the different between these levels is small as 7 dB, therefore a great effect on improving of the signal-to-noise ratio of noisy signals is obtained on receiving, for example, a satellite broadcasting signal. However, in the signal-to-noise enhancer 1, since the phase difference between the first signal path and the second signal path is set to 180 degree by using the delay line 7, an operating frequency bandwidth having a desired enhancement, for example 20 dB or more is narrow as about 100 MHz (Toshihiro Nomoto, Yoshihiro Matsushita, "A Signal-to-Noise Enhancer Using Two MSSW Filters and its Application to Noise Reduction in DBS Reception", IEEE Trans. MTT-41, 8, PP. 1316-1321 (1993)). Thus, the signal-to-noise enhancer 1 can not cover the bandwidth of about 300 MHz which is the bandwidth of all the satellite broadcasting signals received in Japan. Accordingly, the signal-to-noise enhancer 1 can not collectively improve the signal-to-noise ratio in the signals of all the channels for receiving the satellite broadcasting in Japan.
Furthermore, in the signal-to-noise enhancer 1, since the operating frequency bandwidth is narrow as above-mentioned, when the temperature changes, the operating frequency bandwidth changes and easily drifts out of the frequency range desired. Additionally, it is difficult to adjust the operating frequency bandwidth so that the used frequency is within the operating frequency bandwidth, thus, it requires many hours, for example, 2-3 hours to adjust the operating frequency bandwidth.